1. Field of the Invention
The present invention is directed in general to the field of a thin wall thickness container filling and sterilizing system and more particularly to a method for detection of sterilization effect of a hermetically sealed thin wall can containing low acid foods or drinks.
2. Description of the Prior Art
There have been provided various types of canned foods or drinks. Coffee, cocoa, tea, cornsoup, carbonated beverages and the like are filled in cans such as ring-pull cans and pop-top cans, and the cans are hermetically sealed by double-seaming. Such canned foods or drinks are readily available at an automat, for example.
One of problems for many canned foods/drinks manufacturers is to sterilize the foods/drinks contained in hermetically sealed cans. This problem becomes serious when a low acid food, which is generally referred to as having pH value of more than 5, including coffee, cocoa, tea and cornsoup, is to be filled in the can. Since some kinds of bacilli or bacteria, typically "Bacillus stearothermophilus", remains in the low acid food even after being subjected to sterilization at a temperature level of 100.degree. C. or lower, retort sterilization must be applied to the can containing the low acid food at a higher temperature, preferably at around 120.degree. C. or more, for a predetermined period of time, requiring that the can has as its own mechanical characterisitics a high strength and a shape retaining property well resistant to the high temperature sterilization treatment. Consequently, in the prior art hard steel cans (including three-piece steel cans) having wall thickness of about 0.3 mm or more have been used for containing the low acid food.
After the high temperature retort sterilization treatment is applied to the hermetically sealed steel can in which the low acid food has been filled, the can is subjected to a known type of tapping test to confirm if the sterilization is perfectly effected. In case the sterilization effect is not perfect to propagate bacteria in the low acid food, only a skilled man can hear an extraordinary sound or tap tone in the tapping test to realize incompleteness of the sterilization. Such canned foods or drinks are, of course, omitted from commercialization and shipment. Especially in winter season or in cold areas, the can containing a liquid beverage is often heated to a temperature comfortable to drink, say 50.degree. C., at which temperature the Bacillus stearothermophilus and some other kinds of bacilli and bacteria will grow to a tremendous number.
Accordingly, the retort sterilization treatment must be carefully and effectively carried out to completely eradicate the bacilli and bacteria possibly present in the low acid foods.
The above-described conventional method is applied only to a hard steel can. Recent trend is, however, that the hard steel can is replaced by an aluminum soft can having a wall thickness of 0.10-0.15 mm in view of lighter weight, better cost performance and workability, and easier transportation and loading. Moreover, Fe+ ions on the inner wall of steel cans tend to be dissolved in liquid beverages containes therein as the time passes, to thereby deteriorate and degrade the liquid beverage in taste.
When a thin wall soft aluminum can is filled with a liquid beverage, it is required to inject liquified nitrogen into the can, together with the liquid beverage, and immediately after that the can is hermetically sealed by double-seaming. The injected liquid nitrogen is soon gasified to create the internal pressure of the can which will reach a sufficient degree for providing a can strength and maintaining a given shape of the can.
If such a thin wall soft aluminum can is intended to use for containing a low acid food, it will be necessary to apply the retort sterilization at a high temperature of about 120.degree. C. for a predetermined interval, as described hereinbefore. The internal pressure of the can will increase as temperature rises during the sterilization, to more than 1.2 kg/cm.sup.2 G, for example. Such internal pressure level is found to be sufficient to prevent deformation of the can. However, in practice, there has been known no means for detection of completeness of the sterilization. The tapping test is not available for the aluminum soft can because no sensible sound or tone difference can be heard at a pressure level of more than 1.2 kg/cm.sup.2 G.
In the retort sterilization operation, as known, a retort furnace is evacuated to remove oxide gas therein and then a high temperature steam is introduced into the retort furnace, wherein the cans containing low acid foods or drinks are subjected to the retort sterilization at a prescribed temperature for a prescribed interval. If the retort furnace is not effectively evacuated to remain a quantity of oxide gas therein, due to poor thermal conductivity of the oxide gas, a portion of the cans in the retort furnace are not subjected to a steam of the prescribed temperature, resulting in poor and incomplete retort sterilization effect being applied to the foods/drinks containes in said portion of cans. In the event of failure of the steam temperature control system to operate effectively, a desired retort sterilization effect can not be obtained while not attracting the operators' attention to that fact. No method has been known for detecting and confirming completeness of the retort sterilization effect when a soft type can is used and filled with low acid foods or drinks.
Under the circumstances, the thin wall soft aluminum cans have been recognized unsuitable for containing the low acid foods despite their suitability to can materials.